Pipes such as Oil Country Tubular Goods (OCTG) and pipes for pipeline are often used in a connected state at the end portions thereof by a joint part consisting of a so-called pin joint and box joint. Specifically, the end portion of the outer peripheral surface of the pipe is threaded and beveled to form the pin joint. A pair of pin joints formed on a pair of pipes are fastened to the box joint threaded and beveled on the inner peripheral surface thereof, whereby the pair of pipes are connected to each other. If the pin joint has a low dimensional accuracy, the pin joint and the box joint in a fastened state loosen, and therefore, pipes may be disconnected and drop, or in the case of OCTG and pipes for pipeline in which a fluid flows, the fluid may leak from the joint part. For this reason, the requirements for dimensional accuracy and quality assurance level of the pin joint formed in the end portion of the pipe have become severe year by year.
In the dimensional inspection of the pin joint, at present, dimensional measurement is made manually by using a special-purpose measuring instrument to check whether or not the accuracy requirements are satisfied. As the shape of pin joint becomes more complex, the number of dimensional items to be inspected increases, so that much labor and long inspection time are required. Therefore, for a special joint having a complex shape among other pin joints, which is used frequently as the service environment has become harsher in recent years, it has become impossible to perform one hundred percent dimensional inspection on a highly efficient production line for mass-production of pipes.
Therefore, for the special joint of the mass-produced pipe, as a dimensional inspection method consistent with the production efficiency, sampling inspection at fixed frequencies is generally adopted. The pin joint of pipe is generally fabricated by using a CNC lathe. Since the CNC lathe is reliabile for working accuracy, sampling inspection is acceptable on the assumption that only a slow dimensional change caused by the wear of a tool equipped on the CNC lathe occurs. The idea is that if an improper dimension is found in the sampling inspection, the pin joints of pipes included in those that satisfied accuracy requirements in the last sampling inspection before the sampling inspection in which the improper dimension has been found are considered as accepted products to assure the quality as a whole.
Actually, however, it is thought that in addition to the slow dimensional change caused by the wear of the tool, which can be found in sampling inspection, other dimensional changes may be induced by other factors.
Therefore, for the pin joint of pipe, especially the special joint having a complex shape, there has been a need for the development of an automatic dimension measuring device that can take measurements at a high speed and enables one hundred percent dimensional inspection. Conventionally, as a device for automatically measuring the dimension of a thread part of the pin joint and the like, devices described, for example, in JP55-113907A (Patent Literature 1), JP58-205809A (Patent Literature 2), JP61-25001A (Patent Literature 3), and JP2001-293619A (Patent Literature 4) have been proposed.
The dimensions to be inspected of the pin joint of pipe are mainly dimensions such as an outside diameter at a predetermined position in the longitudinal direction with the end face in the longitudinal direction of pipe being a reference. In other words, the dimensions to be inspected of the pin joint of pipe are mainly dimensions at a predetermined position in the direction perpendicular to a plane (reference plane) passing through the endmost point in the longitudinal direction of pipe of the points constituting the pipe end face. As these dimensions, for example, a seal diameter and a thread diameter are cited. As shown in FIG. 6A, the seal diameter is an outside diameter SD of a seal part S at a position separate by a predefined distance SL in the direction perpendicular to a reference plane ES passing through endmost points E1 constituting an end face E of a pipe P. Also, the thread diameter is an outside diameter (an outside diameter on an imaginary line V passing through the tops of threads) TD of a thread part T at a position separate by a predefined distance TL in the direction perpendicular to the reference plane ES of the pipe P.
Unfortunately, a long pipe is generally bent in the end portion thereof. In the case where the pipe has been bent, even if the pipe is placed in such a manner that the longitudinal direction is horizontal in the central portion in the longitudinal direction of the pipe, the direction of the reference plane of the pipe does not coincide with the vertical direction, and tilts with respect to the vertical direction according to the degree and direction of the bend. Therefore, if the dimensions of the pin joint are measured on the assumption that the direction of the reference plane of the pipe is the vertical direction, a measurement error occurs. For example, as shown in FIG. 6B, if the seal diameter is measured on the assumption that the direction of the reference plane of the pipe coincides with the vertical direction (that is, the reference plane of the pipe is ES′), a distance SD′ in the vertical direction at a point separate from the reference plane ES′ by the distance SL in the direction perpendicular to the reference plane ES′ (in the horizontal direction) is measured as the seal diameter, so that a measurement error with respect to the actual seal diameter SD (FIG. 6A) occurs. Similarly, a distance TD′ in the vertical direction at a point separate from the reference plane ES′ by the distance TL in the direction perpendicular to the reference plane ES′ (in the horizontal direction) is measured as the thread diameter, so that a measurement error with respect to the actual thread diameter TD (FIG. 6A) occurs. Therefore, in order to accurately measure the dimensions, such as the seal diameter and the thread diameter, at the predetermined positions in the direction perpendicular to the reference plane of pipe, it is important to accurately grasp the direction and position of the reference plane of the pipe.
To reduce the above-described measurement error, the aforementioned special-purpose measuring instrument includes a member having a contact surface that is butted against the end face (the endmost point) of pipe, and a contactor for measuring the dimensions of pipe at predetermined positions in the direction perpendicular to the contact surface of this member. The dimensional measurement is made manually by using the contactor in the state in which the direction and position of the reference plane have been grasped by causing the contact surface of the member to butt against the end face (the endmost point) of pipe.
Unfortunately, in the conventional automatic dimension measuring devices described in Patent Documents 1 to 4, no proposal has been made concerning the accurate grasping of the direction of the reference plane of pipe, and the dimensional measuring has been made on the assumption that the pipe end portion has not been bent. Therefore, depending on the degree of bend of the pipe, a large measurement error may occur, which presents a problem that the dimension measuring device cannot sufficiently assure the ever tightened dimensional accuracy of pin joint.